Strap tensioning tools employed in connection with securing a tensioned strap loop around a package or other object are commonly of the type wherein overlapping ends of the strap loop are engaged respectively by a feed wheel and an adjacent anvil mounted in the base of the tool. Rotation of the feed wheel advances one strap end while the anvil holds the other strap end stationary to tighten the strap loop about a package.
In a common type of strap tensioning tool currently employed, the feed wheel is mounted from a main frame for bodily movement toward and away from the anvil portion to load and release the overlapping strap ends. Normally, this feed wheel is swingably mounted to provide relative movement between the feed wheel and anvil.
The feed wheel is commonly mounted on a drive shaft and the drive shaft is driven through a ratchet and gear transmission assembly from a drive lever mounted on the side of the tool. The drive lever commonly includes a drive pawl which is normally spring-biased to a position for establishing one-way driving engagement with the teeth of a tensioning gear. This creates a drive relation when the drive lever is swung in one direction and allows the drive lever to be swung back without turning the tensioning gear backwards. Further, retaining pawls are provided for maintaining the tensioning gear against reverse rotation. By employing such a ratchet drive mechanism, the drive lever can be worked back and forth, thereby turning the tensioning gear, drive shaft, and feed wheel until the desired strap tension is achieved.
In the strap tensioning tools commonly employed, the tension level must be sensed by the operator of the tool. The operator senses the tension by the increased resistance in drawing the lever in the direction to increase the tension. When the operator determines that he has achieved the desired level of tension, the strap loop is sealed via any of the common methods, such as applying a crimp fold-over type seal to the overlapped strap ends.
There are two problems with such a tensioning procedure. First, the operator does not know at precisely what tension level the tensioning process should be terminated. Secondly, even if the operator knows the level at which tensioning should be terminated, it is difficult for the operator to sense when he has achieved that level of tension in the strap loop. Obviously, it is desirable to avoid overtensioning the strap loop which can cause the strap to break at a sharp corner of a package or article.
Further, even if the strap loop does not break, the level of tension achieved by the operator in each strap loop varies from package to package. Thus, inconsistent results in applying the tension are common.
In some strapping tools, a slip clutch device is employed. This slip clutch device can be set for a particular tensioning level and causes the driving mechanism to slip when that level is reached. Such clip clutch assemblies, however, are not as durable under typical rugged handling and are subject to wear and subsequent malfunction.